Tolerance to allogeneic grafts can be achieved in a number of rodent models, and long-term graft survival has been reported in a number of non-human primate models, A consistent feature of stable graft acceptance is the control of alloantibody production;conversely, the appearance of increasing alloantibody titers portends graft rejection, Two divergent interpretations have been advanced to explain these observations- first, that the activation of alloreactive B cells and alloantibodies are simply markers of alloreactive T cell activation and that T cells are solely responsible for the loss of the allograft;second, alloreactive B cells and alloantibodies contribute to graft rejection;either directly or indirectly by synergizing with alloreactive T cells or other effector cells, Because of the well-characterized pathogenic properties of B cells and alloantibodies, we favor the second interpretation and hypothesize that the control of alloreactive B cells, in addition to T cells, is central to stable tolerance, Indeed, the unexpected efficacy of B cell-directed immunotherapy in controlling a number of autoimmune diseases and transplant rejection, pathologies initially thought to be predominantly T cellmediated, suggests an important role for B cells in these clinical settings, The fate of alloreactive T cells has been extensively defined in various models of transplantation tolerance, but there is virtually no information regarding the fate of alloreactive B cells, We have addressed this disparity by developing a novel experimental model to visualize the fate of alloreactive B cells, We observed that transplantation tolerance is associated with the deletion of the mature alloreactive B cells, and a sparing/enrichment of the transitional/immature alloreactive B cells, These observations are consistent with recent reports of a correlation between altered B cell subsets and tolerance in non-human primates and enriched B cell markers in bio-marker studies of spontaneously tolerant kidney transplant recipients, Thus the overall goal of our proposed studies is to define the role of B cells in the induction and maintenance of transplantation tolerance, The first specific aim is to define the mechanistic basis for the selective deletion of mature alloreactive B cells, and to test whether this deletion is essential for the development and/or maintenance of allograft tolerance, The second aim is to further characterize the preserved immature/transitional alloreactive B cells and test whether these cells contribute to the development or maintenance of tolerance, We antiCipate that these studies will provide insights into the role of B cells in transplantation tolerance, and contribute to the development of a clinical strategy that induces and maintains long-term graft survival in the absence of pharmacologic immunosuppression,